Removing Existing Anonymous FTP Users

We're using ProFTPD and we've gone thorugh WHM to disable Anonymous FTP (after we have a couple hundred clients setup on a box). After we did this, we edited /etc/proftpd/passwd.vhosts and removed all entries of anonymous from that file.

Then, to test things out, we went to WHM and clicked on Synchronize FTP Users and guess what? All of the anonymous users came back. And Anonymous FTP still works.

What I would like to accomplish here is not to necessarily tweak the proftpd.conf file, because that will potentially get overwritten during an update. I would like to know where these anonymous FTP users are being resynched from when a new account gets created or we run that Synchronize FTP Users script.